Creating a healthy mindset towards performance and learning to cope with stage nerves is so important as music is there to be shared! Here are a few things I have found helpful in my daily practice. 

be well-rehearsed – know that you have put the work in
use deep breathing – this is a way of directly communicating to your body that you are safe. It calms the nervous system and quietens the mind. 
visualise yourself playing and feeling good - remind yourself "I can do this!" when you wake up
get enough rest (if possible!) – to give a good performance you need to be able to be present and rest is important for this. If you have trouble sleeping before a big performance, do your best to accept it and enjoy relaxing activities until you can switch off.
love the music you create – choose repertoire that you enjoy and focus on your love of the guitar as an instrument
view nerves as excitement and remember it is a natural response to the situation. Tell yourself “It’s ok, I’m about to start. I feel excited and anxious but I can continue anyway.” 
focus on gratitude that music is a part of your life 
set a goal for each performance e.g. “I’m going to focus on being present with the music”, “I want to feel good in the hour before, during and after my performance. This night is for me.”, “This performance is about connecting deeply to music and allowing my skills to shine.” "I look forward to connecting with others through music and giving them space to breathe and be entertained or moved emotionally".